1. Field of the Invention
The present invention relates generally to medical devices and method and more particularly to a catheter system having a work element coupled to a drive cable disposed within a lumen of a flexible tubular catheter body.
2. Description of the Background Art
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheromas or plaque, on the walls of blood vessels. Such deposits occur in both the peripheral blood vessels, which feed the limbs of the body, and the coronary vessels, which feed the heart. When deposits accumulate in localized regions of a blood vessel, stenosis, or narrowing of the vascular channel, occurs. Blood flow is restricted and the person's health is at serious risk.
Numerous approaches for reducing and removing such vascular deposits are known, including balloon angioplasty, in which a balloon-tipped catheter is used to dilate a region of atheroma; atherectomy, in which a blade or cutting bit is used to sever and remove the atheroma; spark gap reduction, in which an electrical spark burns through the plaque; and laser angioplasty, in which laser energy is used to ablate at least a portion of the atheroma. In order to facilitate treatment of the stenosis, it is often desirable to obtain a visual image of the interior of the blood vessel within the region of interest. Catheters having imaging elements such as ultrasonic transducers are often used to obtain these images.
In many cases, catheter systems having imaging or interventional devices will include a rotatable drive cable disposed within the flexible catheter body. Catheters of this type will have some type of work element fixed to the distal end of the drive cable. In imaging systems, the work element is typically an ultrasonic transducer or a movable mirror associated with an ultrasonic transducer.
An exemplary catheter system having a rotating ultrasonic transducer coupled to a drive cable is disclosed in U.S. Pat. No. 4,794,931, the disclosure of which is incorporated herein by reference. In this system, a drive cable disposed within a flexible tubular catheter body is used to rotate an ultrasonic transducer about an axis parallel to the catheter body. The ultrasonic transducer is thereby caused to scan a region of the blood vessel in a plane normal to the catheter body.
Another catheter system, which may include an imaging or interventional work element connected to a rotating drive cable, is disclosed in U.S. Pat. No. 5,314,408, the disclosure of which is incorporated herein by reference. This "common lumen" catheter system includes a proximal region having multiple lumens for carrying a guidewire and various work elements and a reduced profile single lumen distal region. The catheter body may be formed from a single tubular member, which extends the entire distance from the proximal end to the distal end, or it may be formed from two or more tubular members which are joined together either in tandem or parallel. A catheter system of this type is advantageous in that the common lumen distal region, having a narrower profile, is able to enter into more constricted areas than would otherwise be possible.
A catheter of the type having a common distal lumen is used as follows. First, a guidewire is advanced into the patient until it lies within a region of interest, typically a region of stenosis within the blood vessel. After the guidewire is positioned, the proximal end of the guidewire is fed into the distal tip of the common lumen and the catheter is advanced into the patient over the guidewire. As the catheter advances into the patient, the guidewire enters and travels through a selected lumen of the multiple lumen proximal region of the catheter body.
Advancement of the catheter into the patient continues until the common lumen distal region lies within the blood vessel in the region of interest. The guidewire is then withdrawn into the proximal region to clear the common lumen. A suitable work element, often a rotating ultrasonic transducer, may then be advanced through the common lumen into the region of interest. Use of a catheter system of this type is described more fully in U.S. Pat. No. 5,314,408.
Marketing demands have created a need for a reduced outer diameter tubular catheter body. It is generally desirable to make the diameter of a catheter body as small as possible to allow the catheter to enter narrow and tortuous regions of the patient's vascular system. As the catheter body size is minimized, the strength of the catheter wall is decreased and the danger that the wall will rupture or collapse may become significant. A reduction in strength can lead to slight closure or collapse of the tubular structure under normal operating pressures incurred during a normal procedure. This type of failure causes an increase in friction on the drive cable, commonly referred to as NURD. The NURD effect can cause the motor powering the drive cable to be overworked causing a failure of the catheter.
To satisfy the market demand for smaller tubular catheter bodies, it would be desirable to replace the common lumen design with a single lumen design without sacrificing strength or pushability. To further meet the market requirements the outer diameter of the drive cable could be reduced, as well. An additional benefit of this reduction would be to improve catheter preparation, especially flushing, due to the increased clearance between the drive cable and the proximal tube design.
While the danger of catheter wall failure is present in any catheter system, it may be particularly troublesome in a common lumen catheter of the type described in U.S. Pat. No. 5,314,408. First, the common lumen is designed specifically to be small in diameter and to be positioned in particularly narrow and twisted regions of the vascular system. Second, the work element in a common lumen catheter not only rotates but also advances lengthwise through the lumen. When the rotating work element encounters a kink in the catheter body, it will impact the wall at the kink and in some cases may cause failure of the catheter body wall.
To avoid such failures, an attachment for a carrier, used to accommodate the work element, was designed that causes the carrier to track the lumen of the catheter body more reliably with less danger of puncturing the catheter wall. Such an improved tracking tip is disclosed in U.S. Pat. No. 5,458,585, the disclosure of which is incorporated herein by reference. While a significant improvement, use of the improved tracking tip can sometimes result in users confusing the solder ball, located at the distal end of the tracking tip, for the working element, when the procedure is viewed via x-ray. This leads to a mistaken determination by the user that the target location has been reached with the work element.
Another possible disadvantage of using the tracking tip is its effect on the shape of the work element carrier. The carrier is configured with the tracking tip coupled to its distal surface and the drive cable coupled to its proximal surface. The work element is by necessity, placed in a cavity between these two ends. As the housing travels through the catheter, bubbles may be created in the cavity. The trough configuration of the cavity causes the bubbles to be forced down upon the work element. The bubbles have a tendency to adhere to the surface of the work element. Since ultrasound does not travel through air bubbles, this phenomenon generally causes a blurred or smeared image to appear on viewing monitors.
The benefit of having a tracking tip in most catheter designs is, in essence, to ensure that a failure of the catheter wall will not occur when the work element housing is forced against it, as occurs when the tubular body is kinked or severely bent. Thus, a desired alternative to having a tracking tip would be realized by creating a system failure that occurs at a location other than the tube wall, such as, in the imaging core assembly. As an example, a reduced diameter drive cable can be incorporated into the tubular catheter body. Should the carrier encounter a kink or bend, the increased clearance between the tube inner diameter and the drive cable makes it possible for the drive cable to buckle or distort in a sinusoidal fashion within the tubular body. This buckling or distortion creates additional friction forces which quickly creates enough drag to cause the imaging core assembly to fail in a safe manner.
Commonly, catheter systems are designed to function as a single, integrated apparatus. Generally, the imaging core assembly, drive cable, and tubular catheter body are bonded together using thermal bonds or adhesives. Once formed the catheter is tested and certified for use in practice. If a catheter fails a performance test or fails in practice due to NURD or for other reasons, the entire device is discarded, regardless of where the failure occurs in the system. Moreover, a user having difficulties crossing a lesion, does not have the option to exchange the short guide wire engagement sheath (low stiffness) for the smaller profile longer guide wire engagement common lumen design (high stiffness). The user in this situation is forced to remove the catheter and begin the procedure again with a new catheter. Thus, creating waste and increased costs.
For these reasons, it would be desirable to provide an improved catheter system for use with internal work elements, such as ultrasonic imaging transducers. It would be particularly desirable to provide a catheter system that permits interchange or replacement of key components in the event that individual components fail or the catheter needs to be reconfigured. It would be further desirable if the drive cable of the catheter were configured to fail in a safe manner should the housing encounter a kink or severe bend which might restrict distal rotation of the cable. It would be still further desirable if the catheter body were formed from materials having improved physical properties, such as increased hoop strength and bonding stiffness. At least some of these objectives will be met by the invention described hereinafter.